"Yes, there is a growth suppression effect with stimulant ADHD medications," Swanson tells WebMD. "It is going to occur at the age of treatment, and over three years it will accumulate."

Whether these kids eventually grow to normal size remains a question. Kids entered the study in 1999 at ages 7 to 9. The current report is a snapshot taken three years later. The 10-year results -- when the kids are at their adult height -- won't be in for two more years.

"The big question now is whether there is any effect on these kids' ultimate height," Swanson says. "We don't know if by the time they are 18 they will regain the height."

The finding appears to end decades of debate over whether stimulant medications affect children's growth. Less than 10 years ago, a National Institutes of Health panel concluded that the drugs carried no long-term growth risk.

That opinion was so widely accepted that the study authors -- who include most of the leading ADHD researchers in the U.S. -- did not warn parents that the study medication might carry this risk.

At the time, researchers thought that any short-term stunting of growth would be made up by a hypothesized "growth spurt" that would occur with continued treatment. But Swanson and colleagues saw no evidence of such a growth spurt.

Another widely accepted theory was that ADHD itself stunted kids' growth. But in a surprise finding, the study found that ADHD kids who do not take stimulant drugs are much larger than kids without ADHD. And these untreated kids continued to grow much faster than kids taking stimulant drugs.

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Swanson says that children who had been taking ADHD drugs before the study began were smaller than kids who had not yet started treatment. Those who first began treatment at the start of the study were normal in size, but grew more slowly than normal kids as the study went on.

After three years, the growth suppression seemed to reach its maximum effect. That's also when the effect of the ADHD drug used in the study -- immediate-release Ritalin three times a day, every day of the year -- seemed to wear off.

"We compared the effect of medication relative to just pure behavioral treatment," Swanson says. "That effect was substantial at 14 months and reduced a bit at 24 months. But at 36 months the relative advantage of ADHD drugs over behavioral treatment is gone."

Swanson and colleagues note that the study did not test the sustained-release stimulant medications that are now the standard treatment for ADHD.

Omar Khwaja, MD, PhD, a neurologist at Children's Hospital in Boston, last year analyzed studies of different ADHD drugs and found strong evidence that ADHD drugs do, indeed, stunt children's growth. In fact, Khwaja and colleagues calculated a growth effect that almost exactly matches the effect seen in the Swanson study.

But Khwaja agrees with Swanson that nobody yet knows what the long-term results of this side effect will be.

"Whether there will be rebound growth at end of puberty, the jury is still out," Khwaja tells WebMD.

"Parents have to be aware that stimulants are an enormous benefit to a lot of children with ADHD, but there is reason to be cautious with all medicines that affect the brain," he says. "Growth monitoring should be standard practice for kids taking these medications."

Swanson and colleagues report their findings in the August issue of the Journal of the American Academy of Child and Adolescent Psychiatry.

Other findings from this large study show that both ADHD drugs and behavioral therapy work in children.